1. Field of the Invention
The present invention relates to flexible magnetic recording media, consisting essentially of a web-like nonmagnetic substrate, a magnetic layer applied to one main side of the web-like substrate and a backing coating formed on the opposite main side of the substrate from a polymeric binder and nonmagnetic fillers as well as supporting pigments.
2. Description of Related Art
It is known that flexible magnetic recording media can be provided with backing coatings containing nonmagnetizable, nonconductive and/or conductive substances. U.S. Pat. No. 3,293,066 states that electrostatic charges on magnetic tapes, which can form in recorders having high tape speeds, can be eliminated by applying conductive backing coatings and in addition the backs of the tapes can be made more hard-wearing by means of backing coatings. It is furthermore disclosed in GB-A 1 197 661 and U.S. Pat. No. 4,135,031 that the winding properties of magnetic tapes can be improved by applying backing coatings having a predetermined surface roughness. Such backing coatings are also known for magnetic cards. EP-A 101 020 discloses special binder mixtures which, particularly with the addition of carbon black, give backing coatings with an excellent adhesive strength, wear resistance and stability under high temperature and humidity conditions.
Such backing layers are of particular importance in video tapes, in particular in those for the home video sector. Thus, for improving the scratch resistance and for reducing the drop out number, inter alia U.S. Pat. No. 4,735,325 proposes a backing coating which consists of carbon black having different particle sizes and of fillers having a Moh's hardness of &gt;8 dispersed in a polymeric binder. In addition to an improvement in the wear resistance and a reduction in the abrasiveness, the proposed backing coatings also serve for reducing the light transmittance of the tape material, which is necessary in particular when such tapes are used on commercial video recorders. For this purpose, EP-A 105 471 proposes a backing coating based on barium sulfate/.alpha.-iron(III) oxide with or without particular carbon black additives.
U.S. patent application Ser. No. 07/888713, filed May 27, 1992 provides transparent backing coatings for magnetic recording media, which coatings meet the requirements with regard to the mechanical properties, such as wear resistance and abrasiveness, as well as having sufficient light transmittance, so that a corresponding magnetic recording medium can also be used for the thermoduplication method (TMD method).
The dispersions of the stated components are prepared in dispersing apparatuses, generally referred to as mills, with the aid of which and by the action of a medium to high shear gradient, depending on the design of the mill, the powdered materials are substantially separated into their individual particles and are covered with a binder and/or wetting agent. Owing to the method of preparation as well as the subsequent working up of the nonmagnetic powder materials, agglomerates composed of individual particles having very different secondary particle diameters are formed. For the production of high quality backing coatings, however, it is necessary for the solids, depending on the properties, to be present for the backing coating in the form of the substantially identically shaped individual particles having a predetermined roughness and uniform distances apart. To achieve this in an economically acceptable time, dispersing machines having a high local energy density, for example stirred ball mills, planetary ball mills, sand mills or attritors, are used, as described in, for example, DE-A 35 26 415 for magnetic layers. However, the known dispersing methods have a particular disadvantage. By introducing very high energy densities and using grinding media in a diameter of from 0.2 to 4.0 mm, usually from 0.6 to 2.0 mm, the pigment agglomerates are substantially divided up into their individual particles and the wettable pigment surface is thus increased. In the case of a constant dispersant and/or binder adsorption layer, the binder adsorption layer becomes thinner with increasing degree of division, with the result that the elastic deformation of the adsorption layers which is required for steric stabilization is no longer ensured and the elastic entropic and osmotic repulsion effects can no longer develop to a sufficient extent. The result is that dispersions, in this case particularly backing coating dispersions, often exhibit instability. This is evident, for example, in the comparison with the gloss of two manually applied coatings, one produced immediately after the end of dispersing and the other a few hours later after the dispersion has been stirred in the storage container. The gloss value of the second manually applied coating is substantially below that of the first, this being associated with an undesirable increase in the layer roughness. This problem cannot be solved by using known and conventional dispersants, such as soybean lecithin, phosphoric acid derivatives of the Gafac type, etc.
However, the constant requirement for higher recording densities and maximum electroacoustic and video values with increasingly small thicknesses of the magnetic layers requires homogeneous, extremely smooth surfaces of the recording media and finely rough, defect-free backing coatings.